The charity is working to help children come to terms with the devastating mental scarring of their experiences, providing specialist support to those showing signs of distress including self-harm, nightmares and bedwetting.

A Syrian refugee girl peeks through a hole in a tent at the Zaatri refugee camp, near the Jordanian border with Syria

The aid agency is also calling for the UN to step up its documentation of all violations of children's rights in Syria and that it should have more resources to do this, so that crimes against children are not committed with impunity.

Speaking after returning from Jordan where he met refugee children, Justin Forsyth, Save the Children's chief executive, said: "No child should ever see the horrors being described on a daily basis to our staff on the ground; stories of torture, murder and terror.

"They need specialist emotional support to come to terms with these shocking experiences and their stories need to be heard and documented so those responsible for these appalling crimes against children can be held to account."

The charity has released Untold Atrocities, a collection of first-hand accounts of the conflict from children and parents receiving help from Save the Children after fleeing Syria.

Syrian refugee children wait before International peace envoy for Syria arrives in the Altinozu camp in Hatay city, located on the border with their violence-racked homeland

The accounts contain graphic details of how children are caught up in Syria's war: witnessing massacres and, in some cases, experiencing torture.

"Dead bodies along with injured people were scattered all over the ground. I found body parts all over each other. Dogs were eating the dead bodies for two days after the massacre." Hassan, 14

“I knew a boy called Ala’a. He was only six years old. He didn’t understand what was happening. I’d say that six-year-old boy was tortured more than anyone else in the room. He wasn’t given food water for three days, and he was so weak he used to faint all the time. He was beaten regularly. I watched him die. He only survived for three days and then he simply died. He was terrified all the time. They treated his body as though he was a dog.”Wael, 16

"They hung me from the ceiling by my wrists, with my feet off the ground. Then I was beaten." Khalid, 15

"There was nothing that they did not use to hurt us." Nur, 9

“World leaders have watched this now for over a year and a half. It is time for action. I feel as though Syria is fading away because of this inhumanity.”

Raed, father

Save the Children has been refused permission to enter Syria to help more children but much of the children's testimony corroborates violations documented by the United Nations and human rights organisations in recent months.

The aid agency is providing specialist help to children to recover from their experiences.

Some have started self-harming as they struggle to come to terms with what they have been through, while others are suffering from nightmares, bedwetting and depression.

Save the Children is on the ground on Syria's borders, providing emotional support to thousands of children who have fled to neighbouring countries, helping them recover from their experiences and rebuild their lives.

The agency has launched an appeal to help fund its work in the region.